1. Field of the Invention
The present invention relates to a switched capacitor filter. More particularly, the present invention relates to a technology of a switched capacitor filter preferable as a loop filter in a feedback system, such as a phase locked loop circuit, a delay lock loop circuit, or the like.
2. Description of the Related Art
Today, more and more functions are being incorporated into a system LSI (large scale integrated), so that the circuit scale of the system LSI tends to increase. Therefore, there is a universal demand for a reduction in each circuit incorporated in a system LSI.
The system LSI is substantially inevitably provided with a phase locked loop circuit (hereinafter also referred to as a “PLL”). The PLL has a constraint that the maximum value of a response time cannot be larger than about one tenth of the frequency of an input clock. Therefore, it is necessary to set the CR (capacitive-resistance) product of a loop filter which is included in the PLL to be relatively large. To achieve a relatively large CR product, the capacitance value of a capacitance included in the loop filter is generally set to be large. Therefore, the loop filter occupies a most portion of the circuit area of the PLL, compared to the other components of the PLL. In the PLL, the lower the frequency of the input clock, the longer the response time. Therefore, the capacitance value of the capacitance included in the loop filter needs to be set to be still larger. As a result, the circuit scale of the PLL further increases. To solve such a problem, the reduction of the loop filter circuit scale is also required.
Use of a switched capacitor filter is considerably effective for the reduction of the loop filter circuit scale. In the switched capacitor filter, a resistance which is a component of the filter is implemented as a switched capacitor, and a considerably large resistance can be obtained using a relatively small switched capacitor. Also, by reducing the amount of an input current, the total capacitance of the switched capacitor filter can be reduced, thereby reducing the circuit scale and power consumption.
However, conventional switched capacitor filters are constructed so that the switched capacitor outputs a voltage which occurs while it samples an input current. Therefore, when the switched capacitor is connected in the input side, i.e., a current is input, a voltage which gradually increases (gradually decreases when the current is negative) is output. When the switched capacitor is disconnected from the input side, i.e., a current is not input, a smoothed constant voltage is output. In other words, conventional switched capacitor filters output a voltage signal without completely smoothing an input signal.
FIG. 12 is a graph illustrating an output voltage of a conventional switched capacitor filter. This graph illustrates changes in an output voltage when a current signal is intermittently input to the switched capacitor filter. As described above, the output voltage has a triangular shape in the presence of an input current and is constant in the absence of an input current. Therefore, the output voltage of the switched capacitor filter contains a high frequency component. As a result, as illustrated in FIG. 13, the output of the PLL is contaminated with spurious harmonics, resulting in a degradation in signal quality.